This invention relates to an improved method of selectively annealing a needle. More specifically, it relates to an improvement in the method of positioning a needle in close proximity with an electromagnetic induction coil and passing current through the coil to selectively anneal a portion of the needle.
U.S. Pat. No. 3,753,412 describes a method of selectively hardening needles. A straight felting needle is positioned in a fixture which is connected to a high-frequency current. An electromagnetic coil is wound about the fixture. The turns of the coil have various diameters and spacings relative to each other. When current is applied to the coil, the needle is heated. The amount of heat applied to a specific section of the needle depends on the diameter and spacings of the coils at that section. Consequently, the needle is selectively hardened.
Unfortunately, the method described in the '412 patent requires the use of a wound coil and the placement of the straight felting needle within the center of the winding. While this method is adequate for a straight needle, it is not practical for a curved needle because of the inability to position a curved needle within the center of the winding of the coil.
The ability to selectively anneal or harden a curved needle is important, especially for the manufacture of curved surgical needles. Curved surgical needles are described in U.S. Pat. Nos. 5,797,961, 4,524,771 and 4,513,747.
A surgical needle is typically attached to a suture during the manufacturing process. Annealing of the needles is generally undertaken for a variety of reasons, one being to facilitate the attachment of the suture to the needle. Typically, the needles are annealed at the portion of the needle commonly referred to as the barrel or channel to soften that portion of the needle to facilitate its attachment to the suture. Maximum softening is often chosen to better control the process. This is necessary due to the difficulty of controlling conventional processes, such as flame annealing or resistance annealing. Flame Annealing is a method of heating a metallic part to a desired temperature for a given amount of time using a flame, in order to reduce the material's strength and improve its ductility. Resistance Annealing is a method of heating a metallic object by applying an electric current through the part, or a region of the part to reduce the material's strength and improve its ductility. Selective annealing, as described in the '412 patent, while often desired, simply cannot be obtained with acceptable certainty for curved needles. As a result, there currently exists an inability to selectively control the portion of the needle which is subjected to the annealing process. This is critical for a surgical needle, since a surgeon expects the finish needle to have consistent strength in the point and body portion of the needle. A precisely selected annealing zone will make it more possible to provide needles which have the requisite consistency in their strength.
In view of the need for a method for selectively annealing a curved needle, and in view of the deficiencies with respect to the prior art for selectively annealing needles, what is needed is an improved method for selectively annealing a curved needle. This improved method would be particularly desirable for the manufacture of curved surgical needles.